For preparing surface-reacted calcium carbonate and its use

1. A process for preparing a surface-reacted calcium carbonate in an aqueous environment, the process comprising the following steps:

a) providing a calcium carbonate;

b) providing 5 wt.-% to 50 wt.-%, based on the weight of calcium carbonate, of at least one acid having a pK a of less than or equal to 2.5, wherein the corresponding acid anion is capable of forming water-insoluble calcium salts;

c) providing 1 wt.-% to 20 wt.-%, based on the weight of calcium carbonate, of at least one water soluble non-polymeric organic and/or inorganic weak acid and/or a hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid, wherein the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is selected from the group consisting of boric acid, citric acid, sodium dihydrogen citrate, potassium dihydrogen citrate, disodium hydrogen citrate, dipotassium hydrogen citrate, sodium dihydrogen borate, potassium dihydrogen borate, disodium hydrogen borate, dipotassium hydrogen borate, and any mixture thereof; and

d) contacting the calcium carbonate of step a) with (i) the at least one acid having a pK a of less than or equal to 2.5 of step b), and (ii) the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid of step c), in an aqueous environment, in the presence of gaseous CO 2 , which is formed in situ or provided from an external source, to obtain a surface-reacted calcium carbonate having a BET specific surface area of at least 30 m 2 /g.

2. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is selected from the group consisting of strong acids having a pK a value of less than or equal to zero at 25° C., and medium-strong acids having a pK a value of between zero and 2.5 at 25° C.

3. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is selected from the group consisting of H 3 PO 4 , oxalic acid and a mixture thereof.

4. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is H 3 PO 4 .

5. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is H 3 PO 4 in a concentration of 20% to 40% (v/v).

6. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is H 3 PO 4 , and the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is citric acid.

7. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is H 3 PO 4 in a concentration of 20% to 40% (v/v), and the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is citric acid.

8. The process according to claim 1 , wherein the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is added in an amount between 1 wt.-% and 15 wt.-%, based on the weight of calcium carbonate.

9. The process according to claim 1 , wherein the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is added in an amount between 1.5 wt.-% and 10 wt.-%, based on the weight of calcium carbonate.

10. The process according to claim 1 , wherein the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid is added in an amount between 2 wt.-% and 5 wt.-%, based on the weight of calcium carbonate.

11. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid and then with the at least one acid having a pK a of less than or equal to 2.5.

12. The process according to claim 1 , wherein the calcium carbonate is simultaneously contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid and the at least one acid having a pK a of less than or equal to 2.5.

13. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and during its addition, the calcium carbonate is also contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

14. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 5% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

15. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 10% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

16. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 20% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

17. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 30% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

18. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 50% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

19. The process according to claim 1 , wherein the calcium carbonate is first contacted with the at least one acid having a pK a of less than or equal to 2.5 and after 80% of the total amount of the at least one acid having a pK a of less than or equal to 2.5 have been added, the calcium carbonate is contacted with the at least one water soluble non-polymeric organic and/or inorganic weak acid and/or hydrogen salt of the at least one water soluble non-polymeric organic and/or inorganic weak acid.

20. The process according to claim 1 , wherein the at least one acid having a pK a of less than or equal to 2.5 is provided in the form of:

(i) at least one H 3 O + -ion provider having a pKa of less than or equal to 2.5, wherein the corresponding acid anion is capable of forming a water-soluble calcium salt, and

(ii) at least one anion, provided in the form of a water-soluble salt or hydrogen salt, wherein the corresponding acid of this at least one anion has a pK a of less than or equal to 2.5 and wherein the anion is capable of forming water-insoluble calcium salts.

21. The process according to claim 1 , wherein the calcium carbonate is further contacted with a compound selected from the group consisting of a sodium silicate, a potassium silicate, a lithium silicate, an aluminium silicate, a synthetic silica, a calcium silicate, an aluminium hydroxide, a sodium aluminate, a potassium aluminate, and any mixture thereof.

22. The process according to claim 1 , wherein the surface-reacted calcium carbonate obtained by the process has a BET specific surface area of at least 40 m 2 /g.

23. The process according to claim 1 , wherein the surface-reacted calcium carbonate obtained by the process has a BET specific surface area of at least 50 m 2 /g.

24. The process according to claim 1 , wherein the surface-reacted calcium carbonate obtained by the process has a BET specific surface area of at least 60 m 2 /g.

25. The process according to claim 1 , wherein the surface-reacted calcium carbonate obtained by the process has a BET specific surface area of at least 70 m 2 /g.

26. The process according to claim 1 , wherein the surface-reacted calcium carbonate obtained by the process has a BET specific surface area of at least 80 m 2 /g.

27. The process according to claim 1 , wherein the aqueous suspension obtained after step d) is dried.